The liquid-solid phases of pulps and suspensions are industrially separated in various ways, i.e. the material can be filtered such that a completely clear condition of the outgoing liquid is not required, or the moisture content of the filtered solid matter may be relatively high. Such is the requirement for instance of the separation of coagulated grains from liquid derived from the denaturation of animal organs in the pharmaceutical industry, or with the filtering of lumpy, solid matter (fragmants of cattle lung, extracted medicinal plant-particles, etc.) from the extract juice in the course of separating the solid-liquid phase (so-called first filtering) following the extraction of animal organs and plants.
In the food industry for instance skin particles and membranes are separated from juice for the production of fruit juice (e.g. in the first filtering of fruit juice) in order to increase the filtering capacity of the apparatus that filters the fruit juice until it is clean, and to reduce failure of the apparatus. The fast and simple filtering of lumpy solid matter has become increasingly significant for environmental protection.
Cleaning of the various agricultural products from the impurities before processing is necessary to avoid clogging of the processing equipment, untimely wear, and possible infection by the impurities. The cleaning prior to processing takes place with washing in washing machines. This is the simplest way to remove soil and remnants of spraying from the surface of the produce, as well as a considerable part of the substances capable of infecting the surface (e.g. as in the cleaning of potatoes prior to processing for distillation, in starch production, in preliminary washing of sugar-beet, tomato, fruits, etc.).
Valuable materials can be removed from the tissue or marc of the pulp derived from crushing of the agricultural products, and the tissue can be separately collected. For instance in potato starch production washing sieves are used to separate the starch of the pulpy potato from the marc.
Several methods are known for the gravitational first filtering of suspensions and pulps and for the simple extraction of liquid from the solid materials, some of which can be briefly described as follows:
Prismatic or cylindrical filters have a filter body with a slanting filter frame surrounded with flat plates, or with surfaces of revolution. This is rotated at uniform angular velocity. The pulp is admitted into the interior of the filter body. The filtrate passes through the filter, while the retained solid matter--as a result of the joint effect of rotation and slope--moves forward to the outlet.
In one of the variants of this equipment the sieve cylinder rotates in a semicylindrical trough. The pulp is admitted into the trough, from which the filtrate passes into the interior of the cylindrical filter body through the filter mantle of the latter, and from there is carried off through a pipe. The solid matter is removed from the exterior of the cylinder by a revolving brush extending into the trough. From there the solid matter is removed with the aid of a mechanical device.
Another group of the separators is represented by the flat filters. Depending on the method of moving the filter surface there are horizontal-, at an angle to the horizontal-, and rotary flat filters, as well as swinging-vibrating filters moving to and fro in vertical plane, on a circular, or elliptical path, or along a straight line.
The filter surface is suspended with a hinge at a low angle. The pulp is carried onto the filter surface, the filtrate passes through, while the retained solid matter slides to the outlet orifice. For cleaning the filter surface of the swinging-vibrating filters a screen with large holes is provided at a certain distances below the filter, the screen-carrying large diameter balls. As a result of the swinging-vibrating motion, the balls clean the surface by constantly knocking the filter surface from underneath.
For simple separations e.g., in sewage purification, various strainers (first filtering) are used, which are arranged horizontally or at an angle. The strainers can be curved (sweep filters). The pulp is carried to the upper part of the vertical slanting, or curved strainer. The filtrate passes through the strainer while the solid matter slides down on the sloping (curved) path to the mechanical discharge mechanism.
There are also skew-walled, filter bags which cooperate with a delivery worm in a lower rising part of the filter bag. The pulp passes into the space surrounded by filter walls on both sides, the filtrate passes off through the filter apertures, while the retained solid matter slides down on the flat wall and is removed by the worm.
Several methods are known for the washing-cleaning of solid lumpy materials and for the dewatering thereof:
One such washing machine has a horizontal trough with double bottom. The upper bottom is a semicylindrical perforated plate with several recessed stone traps. A shaft is provided with spiral arms running along the centerline of the washing machine. These arms carry the lumpy solid matter in the longitudinal direction of the machine. The washing liquid is drained into the trough, the liquid containing the impurities passes off through the perforated plate of the upper bottom to be collected in the lower bottom. The washed cleaned solid lumpy material is removed by the perforated baskets at the end of the shaft.
There are also washing mechanisms where the washing trough is divided into two or three parts in its longitudinal direction with cross walls. In this case the lumpy solid matter is carried from the inlet by the specially formed rotary arms by lifting from one cell into the next one. Each cell is provided with a separate water inlet and outlet so that the lumpy cleaned material is supplied with fresh washing liquid in every cell.
Another washing mechanism has a vertical cylinder in which the perforated rotary spiral surface lifts the lumpy material to be cleaned at uniform rate and meanwhile it is washed down by the liquid admitted from the top. The space required by this apparatus is relatively small.
A belt-conveyor type washing mechanism is generally used in the food industry. Here the material to be washed (e.g. soft, delicate fruit) is first passed under water, then the material emerges from the region of the sprinklers placed at the bottom and top. Next it passes into a perforated elevator to be washed again with clean water.
A rotary cylindrical washing mechanism is also known in the fruit-processing field in which the material to be washed is carried into the flowing washing liquid. The mechanical impurities (sand, etc.) fall through the holes of the drum, while the floating materials (e.g. stems, etc.) pass off on the water surface. For washing of hard fruits a conveyor-arm type washing mechanism is used, in which baffle plates rotate and carry the fruits. Soft fruits are washed and carried by flowing or injected water from the inlet to the outlet.
The more familiar washing machines for technological purpose, namely for the separation of the solid-liquid phases of pulpy materials, are the following:
A vertical cylinder mantle of a cylindrical washing sieve is covered with sieve cloth. A horizontal shaft fitted with arms runs through the machine. These arms lift up the pulp to be washed and guide it under the shower of the washing water. Here the pulp is washed over and falls back onto the sieve, the surface of which is cleaned with constantly rotating brushes. The water containing the valuable material (e.g. starch) passes through the sieve, while the hairy and fibrous tissues are retained to be carried off with the aid of a worm.
Cylindrical rotary sieves are also known which are similar to the centrifugal pump. The blades of the impeller are provided with an inlet screen. The pulp arrives through the tubular shaft onto the sieve plate and the liquid containing the valuable material passes through the holes of the rotary blades into the housing. The tissue remains on the sieve plate and, due to the effect of the centrifugal force they pass through the blades, moving off when the periphery is reached.
A frame of a vibrating washing sieve can be flexibly fixed (with the aid of springs). The to and fro motion and the vibrating motion are brought about by electrical-mechanical device. The surface of the sieve is divided into several fields, and the sieve surfaces slope toward the outflow. A shower is provided over every field and the liquid carrying the valuable material is retained by a tray at the bottom, while the tissue leaves the filter surface through the separately mounted channel.
The systems described above are used for specific filtering or washing task.
However simple equipment suitable effectively and with high capacity for filtering of sticky, solid materials from liquid, or dewatering without damaging the surface of the solid matter and without pressing it through the filter, has thus far not been known.